[2] Llangefni Town were champions in the previous season and were promoted to the newly formed Cymru North.
The bottom two teams from the previous season, Barmouth & Dyffryn United and Llandudno Junction, were relegated to Division 2 for 2019–20.
In Division 2 champions, Glan Conwy and runners-up, Blaenau Ffestiniog Amateur were promoted in their place.
Glan Conwy were champions in the previous season and were promoted to Division 1 along with runners-up, Blaenau Ffestiniog Amateur.
On 21 July 2019, Llannerch-y-medd resigned from the Welsh Alliance League after losing the entire squad of players from the previous season for various reasons.